The present invention provides a composition for the preparation of bone cement.
Bone cements are used for, among other things, securing implants, anchoring artificial members of joints, in restoration surgery of the skull, and for joining vertebrae (discs). Plastic pastes which when cured form bone cements are produced by blending polymerized materials consisting of homopolymers or copolymers of methylmethacrylate together with suitable liquid monomers, primarily methylmethacrylate, and a catalyst system. The plastic pastes may also include additives which, when the bone cement is cured provide contrast when the portion of the body including the bone cement is x-rayed, e.g., zirconium dioxide. They may also include such adjuvants as dyes which permit observation of the cemented portion in the body. This plastic mass is inserted in the body, usually to join natural or artificial bony portions and is cured (hardened) by polymerization of the monomer. Redox catalyst systems are used which comprise an organic peroxy compound, usually dibenzoylperoxide as the catalyst, with a reducing agent (accelerator), e.g., dimethyl-p-toluidine, of which the correct chemical nomenclature is N.N-dimethyl-p-toluidine.
German patent publication No. AS-2,229,702 according to British Pat. No. 1,431,211 discloses a bone cement composition consisting of polymethylmethacrylate and a monomer mixture of methylmethacrylate and esters of methacrylic acid with higher alcohols, and a catalytic system consisting of dibenzoylperoxide and dimethyl-p-toluidine.
German disclosure document No. OS-2,501,683 discloses a composite material and its use as a bio-active bone cement. This composite material is a synthetic matrix based upon methacrylate containing a bioactive substance which stimulates the growth of the bone, and which may be a glass ceramic having an apatite crystalline phase consisting of 20-60% by weight SiO.sub.2, 5-40% of P.sub.2 O.sub.5, 2.7-20% of Na.sub.2 O, 0.4-20% K.sub.2 O, 2.9-30% of M.sub.g O, and 5-40% of CaO. The said glass ceramic is disclosed in U.S. Pat. Nos. 3,981,736 and 3,922,155.
A fibrous material, for example, glass fibers, can be added to the composite material disclosed in German disclosure document No. OS-2,501,683 to improve its mechanical characteristics. When using such a composite material as a bone cement, for example, the hardner is added to the liquid methylmethacrylate, and then the finely powdered bio-active glass ceramic is added. When a pliable and malleable mass has been formed as a result of polymerization, the outer surface of the mass may be enriched with additional powdered glass ceramic, and then the mass may be inserted into the body and cured. In contrast with those bone cements which are produced from mixtures of polymers and monomers, the time needed for the polymerization to form the pliable mass in case of the bone cement containing the glass ceramic as disclosed in German disclosure document No. OS-2,501,683, which does not contain any polymers, is unduly long, precluding its use in surgical practice. The known bone cements containing a mixture of polymers and monomers have the advantage that the time required, after mixing thereof, for the development of a mass having a viscosity sufficiently high that it is suitable for application to the bone is considerably shorter.
The known bone cements containing the glass ceramic material contain a higher content of monomers than the conventional bone cements containing mixtures of polymers and monomers. This is a disadvantage because of the greater danger that monomer constituents of the bone cement may be carried into the blood stream.
It is an object of the present invention to provide a composition containing a mixture of copolymers of methylmethacrylate and methylacrylate useful when cured after mixing with the liquid monomers as a bone cement having advantageous processing characteristics, good mechanical properties when cured, and characteristics favorable to the development of bone structure.